Uninephrectomized rats were treated appropriately to create DOCA-salt hypertension and water drinking, saline drinking or DOC injected controls. Thymocytes were used to measure sodium transport activity in buffer and in serum. DOC salt hypertensive rats possessed higher blood pressure than controls and greater values of total and ouabain insensitive rate constants for sodium efflux. Serum caused an elevation of all components of sodium transport in both hypertensive and control rats but did so to a greater extent in the hypertensives. Serum from DOCA salt hypertensive rats contains a pump stimulant rather than an inhibitor.